Researchers in England have created a prototype surveillance device that can be used to spy on people inside buildings and behind walls by tracking the frequency changes as Wi-Fi signals generated by wireless routers and access points bounce off people as they move around.

The device, which is about the size of a suitcase and has two antennae and a signal processing unit, works as a "passive radar system" that can "see" through walls, according to PopSci.com. It was able to successfully determine the location, speed, and direction of a person behind a one-foot-thick brick wall, but can not detect people standing or sitting still, the article said.

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The U.K. Ministry of Defence is looking into whether the device -- designed by Karl Woodbridge and Kevin Chetty of the University of College London -- can be used in "urban warfare" for scanning buildings, PopSci reported.

A series of experiments was conducted which involved personnel targets moving inside a building within the coverage area of a WiFi access point. These targets were monitored from outside the building using a 2.4-GHz passive multistatic receiver, and the data were processed offline to yield range and Doppler information. The results presented show the first through-the-wall (TTW) detections of moving personnel using passive WiFi radar. The measured Doppler shifts agree with those predicted by bistatic theory. Further analysis of the data revealed that the system is limited by the signal-to-interference ratio (SIR), and not the signal-to-noise ratio. We have also shown that a new interference suppression technique based on the CLEAN algorithm can improve the SIR by approximately 19 dB. These encouraging initial findings demonstrate the potential for using passive Wi-Fi radar as a low-cost TTW detection sensor with widespread applicability.

Given the concerns over the government's use mobile body scanner technologies, the development of a device that allows someone to snoop on a person's movements within his or her own home will no doubt be met with public outcry.

Updated 3:07 p.m. PTto correct name of agency interested in the technology to U.K. Ministry of Defence.

About the author

Elinor Mills covers Internet security and privacy. She joined CNET News in 2005 after working as a foreign correspondent for Reuters in Portugal and writing for The Industry Standard, the IDG News Service, and the Associated Press. E-mail Elinor.
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